"Why Don't We Have..." is a PopMech series that quickly explains why some of the technologies promised by science fiction have yet to become fact. Today: faster-than-light travel.

Faster-than-light travel is one of those little necessities of sci-fi. Space is so vast that without such ludicrous speed our heroes would never interact with anyone. So Star Trek's Enterprise, Star Wars's Millennium Falcon, and the Battlestar Galactica all travel faster than the speed of light.

With modern propulsion technology, humans can't even begin to approach a speed that would make interstellar travel feasible. But the biggest obstacle for FTL is the one kids learn in school: light speed is the galactic speed limit. Writing about the issue on NASA's website, David Allen Batchelor says that FTL travel "violates known physics (Einstein's Special Theory of Relativity) to an extent that can't be defended."

Yet plenty of imaginative physicists don't want to take no for an answer, and have turned to innovative (if bizarre) theoretical ways around the problem. In 1994, for example, Miguel Alcubierre imagined a bubble of normal space–time around a spacecraft that would allow for FTL travel by expanding space itself behind the ship and compressing it in front. Normal physics still applies within the bubble, and so the passengers don't feel any acceleration at all. Rather than violating known physics, the Alcubierre drive actually conformed to the General Theory of Relativity.

You can't get in or out of the bubble. Crossing the threshold would probably crush and obliterate a spaceship.

You can't destroy the bubble. One must deform space–time to make the bubble in the first place. How would you re-form it? No one knows.

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The Alcubierre drive concept. Credit: NASA

You might destroy your destination. Even if scientists knew how to stop the bubble once it started, decelerating would release high-energy particles from the front-facing side. Anything at the destination "would be high-energy-particle blasted into oblivion," according to a paper from the University of Sydney.

It requires energies that might be impossible (or incredibly difficult) to achieve. In mathematical models, the sides of the bubble would require energy per unit of volume to be less than zero. We're not sure this "exotic energy" exists. And if it does, it would take 10 billion times the mass of the observable universe, converted into negative energy, to power Alcubierre's idea. Recently Chris Van Den Broeck tweaked the model and made it more energy efficient but still just about impossible.

It's a pretty daunting slate of challenges. But that doesn't mean NASA's not trying—the agency announced last fall that its scientists are at work on a way to tweak the design of Alcubierre drive to drastically cut its energy requirements.

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